1. Field of the Invention
The invention involves a process for recording semihalftone images. In this process, an original is reproduced on a recording medium by individual recording elements with the tone value "print" or "no-print". The original is divided into multiple scan fields comprising individual scan elements and each scan field is subdivided successively into the smallest subfield of a predetermined size.
2. Description of the Related Art
With this type of process, originals are reproduced on a recording medium, for example, a sheet of paper, a printing plate or a fluorescent screen, by means of individual equal size recording dots, which are either completely or not inked. In this process, the image information in the original is collected by a sensor or scanning device, for example, a scanner. For each image dot (scan dot), the scanner emits a signal, which corresponds to the tonal value (in color images, the color tone) of the scan dot. In a black and white original, the gray tones, which range from black to white, are realized on the recording medium by the frequency of the black recording dots being varied as a function of the desired gray tone. All recording dots in a black area of the original are black in the reproduction, while the proportion of white recording dots located among the black recording dots increases as the blackness decreases, that is, the frequency of the black recording dots is lower. In white areas of the original, all recording dots are white, that is not inked. In this way, the gray scale can be reproduced in multiple, discrete steps, that is, semicontinuously. This type of reproduced image is also referred to as a "semihalftone" image in which the tonal values of the tonal value scale are reproduced according to a pre determined subdivision into tonal value gradations.
The processing of such semihalftone images with the assistance of digital technique is very simple, because the image processing can proceed by the binary method ("1" or "L" for a print dot and "0" for a no-print dot). Thus, the semihalftone image corresponds to a binary record of the image information in an original, which, therefore, can be easily recorded on a storage medium, such as magnetic tape, a magnetic disk or a diskette, but also can be recorded, for example, in a semiconductor storage device.
Within the skill of the art, the word "recording" means any type of imaging of the original as a semihalftone image. This concept includes particularly the reproduction of the original on a sheet of paper or on a fluorescent screen, the preparation of a printing plate suitable for reproduction of the original and storing the image in a storage device.
A process for the binary recording of images is known from U.S. Pat. No. 4,578,713. In this process, an original is scanned linewise; the tonal value of each scan dot is determined and stored. The original is subdivided into multiple scan fields, each scan field comprises multiple scan dots, that is, the tonal value data for the scan dots of the original are subdivided into data blocks. The average tonal value of each data block is calculated from the appropriate tonal value data. From this is determined the number of recording dots required to reproduce the average tonal value. The distribution of the recording dots to be printed per scan field is determined by successive subdivision of the scan field into the smallest subblocks. For the smaller data blocks resulting from each subdividing step, the average tonal value and the number of related recording dots to be printed to yield this average tonal value is determined from the tonal values of the pertinent scan dots. The known process, in order to determine the number of recording dots to be printed within the subfields, begins with examining the scan field itself or the largest subfields respectively and continues by considering smaller and smaller subfields, until the scan field is finally subdivided into the individual scan dots. In addition to the determination of the number and the distribution of the recording dots to be printed, an error calculation must also be performed in each step. This becomes more and more costly with increasing subdivision, because the number of data subblocks increases from one subdivision step to another and quadratically at that. Because of these ever-increasing error calculations, the advantages of digital image processing are diminished.